Temperature-activated polysilicic acids and their use in paper production processes

ABSTRACT

A temperature-activated polysilicic acid is prepared by heating an acidified aqueous alkali metal silicate. The temperature-activated polysilicic acid is used in the production of paper for improvement of retention and water removal.

This application is a division of application Ser. No. 08/375,886 filedJan. 20, 1995 now U.S. Pat. No. 5,571,494.

TECHNICAL FIELD

The present invention relates to a temperature-activated polysilicicacid prepared by heating an acidified aqueous alkali metal silicate. Thepresent invention also relates to a process for the production of paperutilizing the temperature-activated polysilicic acid for improvement ofretention and water removal.

BACKGROUND OF THE INVENTION

During the commercial production of paper, a uniform and continuousstream of a dilute water suspension comprising pulp fibers and otheradditives are allowed to flow readily through a narrow slot onto atraveling wire mesh. Most of the water drains rapidly through the wiremesh as it moves over a series of table rolls and suction boxes whichsupport the wire and remove water from the forming sheet. As more wateris drawn from the wet sheet, the consistency of the sheet graduallyincreases to about 20% at which point the paper sheet is strong enoughto be transferred to a porous fabric blanket where it is pressed andlooses water. Most of the remaining water, more than half the dry weightof the sheet, is evaporated out of the paper leaving about 5% to 8%moisture in the paper to minimize dimensional changes which occur as thepaper establishes equilibrium with the relative humidity of the air inthe production plant.

The addition of various chemicals to improve the properties of paperhave been described in the prior art. For example, U.S. Pat. No.4,954,220 to Rushmere relates to the use of anionic polysilicatemicrogels in combination with an organic polymer to flocculate pulp andpapers fines in a papermaking operation. The polysilicate microgels areformed by the partial gelation of an alkali metal silicate or apolysilicate such as sodium polysilicate. The microgels are referred toas active silica in contrast to commercial colloidal silica and consistof aggregates of very small particles, e.g., 1 nanometer, and formthree-dimensional networks. The microgels are described as having veryhigh surface areas and are used in the papermaking process inconjunction with cationic polymers. In the section of the patent under"Detailed Description of the Invention", reference is made to thedescription of active silicas in the book "The Chemistry of Silica"published in 1979 by R. K. Iler as providing a disclosure of activesilica microgels which can be used in the patented invention. The patentthen sets forth a series of alternative methods by which polysilicatemicrogels may be prepared, including: 1) aqueous solutions of alkalimetal silicates which have been acidified by acid exchange resins; 2)inorganic and organic acid aqueous solutions of alkali metal silicateswhich have been acidified by acids, salts and gases; 3) aqueoussolutions of alkali metal silicates to which alkali metal salts ofamphoteric metal acids have been added; and 4) alkali metal silicates towhich certain organic compounds have been added. The patent thenconcludes in column 3, lines 50-55 that the simplest and most economicmethods for the preparation of polysilicate microgels are productsproduced by acidification of sodium polysilicate solutions with a commonmineral acid or addition of a gel initiator such as alum, sodium borateor sodium aluminate to a sodium polysilicate solution.

EKA Nobel PCT Published Application WO-90/107543 discloses a process forthe production of paper in which three components are added to the papersuspension, a cationic starch, a cationic polyacrylamide, and apolymeric silicic acid, to improve retention and water removal in paperproduction. This PCT application discloses that the polymeric silicicacid is the same product as disclosed in Swedish Patent Application No.8801951-8, which product has a very high specific surface area in therange of 1100 to 1700 m² /g, and can be prepared by acidification of analkali metal silicate such as sodium waterglass, the acidification beingconducted by use of a mineral acid such as sulfuric acid or acid ionexchangers. In Example 1 on page 7, the polymeric silicic acid wasprepared from waterglass which was diluted with water to an SiO₂ contentof 5% by weight, after which the aqueous solution was ion exchanged withion exchange resin Amberlite IR-120 to a pH of 2.3.

EKA European Patent Application No. 041056, directed to a paper makingprocess, is disclosed in column 1 of the EKA Nobel published PCTapplication. This published application discloses a papermaking processwhich uses a binder of colloidal silicic acid and cationic starch addedto the stock for improving the paper or retention of the stockcomponents. Colloidal silicic acid is described, for example at page 7,as being a known material made by reacting waterglass with sulfuric acidby known procedures to provide a silica having molecular weights up toabout 100,000. However, the patent states that this product is unstableand then points out at page 8 that by the invention disclosed in theEuropean Patent Application, superior results are obtained through usewith the cationic starch, of a colloidal silicic acid in the form of asol, wherein the colloidal silicic acid in the sol will have a surfacearea of from 50-1000 m² /g and preferably 200-1000 m² /g. Thepublication states that such silicic acid sols are available fromvarious sources including Nalco, DuPont and EKA.

At page 20, Example IV, there is a specific description for producing acolloidal silicic acid component wherein 100 ml of waterglass werediluted with 160 milliliters of water and slowly fed into 130milliliters of 10% sulfuric acid under vigorous agitation. When all thewaterglass had been added, the pH was 2.7 and the SiO₂ content was 8% byweight. This acid sol was diluted to 2% SiO₂ by weight, added to anEnglish China Clay grade C followed by addition of 2% cationic starchsolution. These mixtures were then added to the paper.

Finnish Application No. 68283 discloses a two-component system used inthe paper making process. The system comprises a cationic starch and acolloidal silica sol. This system is intended for improving retentionand strength properties of the paper product. However, thistwo-component system is not commercially attractive due to the highprice of the colloidal silica sol.

Finnish Application No. 894362 proposes a two component system in whichcolloidal silica sol is replaced with a certain anionic polysilicatemicrogel, which is prepared by a partial gelation of an alkali metalsilicate by adding certain initiators to an aqueous solution of thealkali metal silicate.

None of these prior publications recognize that a temperature-activatedpolysilicic acid prepared by heating an acidified aqueous alkali metalsilicate has improved properties as a retention agent in paperproduction. Further, none of these publications disclose a process forthe production of paper utilizing a temperature-activated silicate forimprovement of retention and water removal.

The present invention provides a novel temperature-activated polysilicicacid prepared by heating an acidified aqueous alkali metal silicate. Useof the novel temperature-activated silicate in the paper productionprocess represents a significant improvement over the prior art for thefollowing reasons: 1. They improve the mechanical properties of thefinished paper; 2. They improve the recovery of fines and colloids fromthe water cycle; 3. They improve filterability; 4. They improve grammageand ash retention, i.e., filler retention; and 5. They are economicaland easy to use.

SUMMARY OF THE INVENTION

The present invention provides a novel temperature-activated polysilicicacid agent prepared by heating an acidified aqueous alkali metalsilicate.

Another aspect of the present invention relates to a process for theproduction of paper comprising the steps of:

adding components comprising a temperature-activated polysilicic acidand a cationic polymer to an aqueous fiber pulp suspension; and,

carrying out sheet forming and water removal.

The present invention further provides a method for improving retentionin a papermaking process wherein a temperature-activated polysilicicacid is added to an aqueous fiber pulp suspension.

Modifications are possible within the scope of this invention.

DESCRIPTION OF THE INVENTION

This invention will be described in detail with reference to thepreferred embodiments and further illustrated by the examples provided.

The present invention relates to a temperature-activated polysilicicacid prepared by heating an acidified aqueous alkali metal silicate. Useof this novel temperature-activated polysilicic acid in a papermakingprocess results in the production of paper having superior properties.The present invention is based, in part, on the inventor's unexpecteddiscovery of a temperature-activated polysilicic acid prepared byheating an acidified aqueous alkali metal silicate. The alkali metalsilicate can be acidified to a broad pH range, from about 0 to 5, andmore preferably to a pH range from about 0 to 2. Generally, when thealkali metal silicate is acidified to a pH near or below the isoelectricpoint of the alkali metal silicate, i.e., about 2.0, the polysilicicacid no longer functions well as a retention aid in the paper productionprocess. It should therefore be appreciated that the present inventionis also based on the inventor's surprising discovery that when anaqueous alkali metal silicate is acidified to a pH near or below theisoelectric point of the alkali metal silicate and is then heated forvarying time periods to produce a temperature-activated polysilicicacid, the temperature-activated polysilicic acid greatly improves thepapermaking process by producing paper with superior fiber and ashretention levels. Moreover, the temperature-activated polysilicic acidof the present invention has increased adherence efficiency over priorart agents. Thus, the temperature-activated polysilicic acids of thepresent invention combine and adhere strongly with only one cationicpolymer. This eliminates the need to use two or more cationic polymersas in many of the prior art systems.

The temperature-activated polysilicic acid used in the present inventionhas a very high surface area of approximately 1350-1600 m² /g. The givenspecific surface area is measured by the titration method of G. W.Sears, Anal. Chem. 28, (1956), p. 1981. The temperature-activatedpolysilicic acid of the present invention is prepared by acidificationof an alkali metal silicate, such as potassium or sodium silicate,preferably sodium silicate, followed by heating. Potassium and sodiumsilicate are available with varying molar ratios of SiO₂ to Na₂ O or K₂O and the molar ratio is usually within the range of from 1.5:1 to4.5:1, preferably 3.3:1, and the alkali metal silicate usually has anoriginal pH around 13. Waterglass is especially preferred.

Mineral acids, such as sulfuric acid, hydrochloric acid and phosphoricacid are used to acidify the aqueous alkali metal silicate. Theacidification is carried out to a pH within the range of from about 0 to5 and more preferably to a pH within the range of from about 0 to 2. Theaqueous alkali metal silicate may be acidified by slow addition of adiluted alkali metal silicate solution into a mineral acid.Acidification with an ion exchange agent or organic acids, such asacetic acid, may also be used.

The SiO₂ concentration in the aqueous alkali metal silicate solution tobe heated is preferably about 1-20 weight percent. When necessary, thetemperature-activated polysilicic acid agent obtained after heating isdiluted with water to a desired concentration.

The aqueous alkali metal silicate solution is heated to a temperature ofabout 30°-250° C. and more preferably to a temperature of about 60°-90°C., for a period of about 1-120 hours, more preferably about 1-20 hours.The heating may be done by using the secondary temperature of a papermill in a tank adjacent the papermaking machinery. The heating may alsobe done under pressure necessary to achieve the desired temperature.

After heating, the resulting temperature-activated polysilicic acid maybe mixed at room temperature and diluted, more preferably mixed at roomtemperature for 72 hours and diluted to 0.15% SiO₂.

Another aspect of the present invention relates to a process for theproduction of paper comprising the steps of:

adding components comprising a temperature-activated polysilicic acidand a cationic polymer to an aqueous fiber pulp suspension; and,

carrying out sheet formation and water removal.

The fiber pulp used in the suspensions of the present invention may bederived from any fibrous raw materials including hardwood, softwood,nonwood, straw, bamboo, hemp or recycled paper, board, or mixtures fromdifferent fibrous raw materials. Moreover, the fibers may be obtained byany process known to those skilled in the papermaking art includingmechanical or chemical pulping such as refiner groundwood, kraftpulping, sulfite pulping or mixtures of pulp obtained from differentmethods.

Cationic polymers useful in the present invention include cationicstarches, cationic guars and cationic polyacrylamides. Cationic starchesare particularly useful in that they are inexpensive and impart drystrength to the paper. The amount of cationic polymer to be added to thefiber pulp suspension is preferably about 0.005 to about 5 wt. %calculated from dry fiber.

The amount of temperature-activated polysilicic acid and cationicpolymer used in the process for paper production according to thepresent invention can vary within wide limits depending among otherthings on the type of stock, the presence of fillers and otherconditions. The amount of temperature-activated polysilicic acid,calculated as SiO₂, that may be added to the fiber pulp suspension isabout 0.005-10 weight percent, more preferably about 0.05-5 weight %,calculated from dry fiber.

Additional materials may be added to the pulp to improve the propertiesof paper for particular uses. Examples of such additional materialsinclude dyes which may be added to produce colored paper; fillers suchas kaolin or other clays, calcium carbonate, silicon dioxide, syntheticsilicates or titanium dioxide may be added to increase the density ofthe sheet, to decrease its porosity, and to increase its opacity. Waxmay be impregnated into the paper, or a polyethylene or other polymerfilm may be applied to the surface, for water proofing. Bentonite may beadded to the fiber pulp suspension before the water removal stage tofurther improve retention in the system.

While the present invention is disclosed generally above, preferredembodiments are further discussed and illustrated with reference to theexamples below. However, the following examples are presented toillustrate the invention and should not be considered as limitations.

EXAMPLE 1

In this example, a temperature-activated polysilicic acid according tothe present invention is prepared.

60 ml of 3.3 MR (molar ratio) sodium silicate solution was diluted with290 ml of deionized water. The 3.3 MR sodium silicate was Zeopol® 33,sold by Zeofinn Oy of Finland. This solution, which had a concentrationof about 6% SiO₂, was added slowly into a solution of 248.5 ml ofdeionized water and 48.5 ml of 10% sulfuric acid under strong agitation.The final silica concentration was 1.5% Sio₂. This solution was heatedat 60° C. for 20 hours and then diluted to a silica concentration ofabout 0.15%. The pH of the final solution was 1.4.

EXAMPLE 2

In this example, a temperature-activated polysilicic acid agent of thepresent invention is prepared.

60 ml of 3.3 MR sodium silicate solution was diluted with 290 ml ofdeionized water. This solution, which had a silica concentration ofabout 6% was added slowly into a solution of 248.5 ml of deionized waterand 48.5 ml of 10% sulfuric acid under strong agitation. The finalsilica concentration was 1.5% SiO₂. This solution was heated at 60° C.for 5 hours and then diluted to a silica concentration of 0.15%. The pHof the final solution was 1.4.

EXAMPLE 3

In this example, a temperature-activated polysilicic acid according tothe present invention is prepared.

60 ml of 3.3 MR (molar ratio) sodium silicate solution was diluted with290 ml of deionized water. The 3.3 MR sodium silicate was Zeopol® 33,sold by Zeofinn Oy of Finland. This solution, which had a concentrationof about 6% SiO₂, was added slowly into a solution of 248.5 ml ofdeionized water and 48.5 ml of 10% sulfuric acid under strong agitation.The final silica concentration was 1.5% SiO₂. This solution was heatedat 60° C. for 20 hours and then mixed at room temperature (23° C.) for72 hours before it was diluted to a silica concentration of about 0.15%.The pH of the final solution was 1.4.

EXAMPLE 4 Fine Paper Trial--Comparative Tests

This example compares the performance of the temperature-activatedpolysilicic acid agent prepared in Example 1 and Example 2 (with starchor polyacrylamide) with that of Compozil (colloidal silica and cationicstarch) and Hydrocol (bentonite and polyacrylamide).

Bleached softwood pulp was used with calcium carbonate as a filler. Thecalcium carbonate was Snowcal 75, sold by Omya Ltd. The pulp wasreceived from a pulp mill in Finland as a bale and was free of chlorine(TCF-pulp). The pulp was ground in a Valley-hollander to SR-number 35according to method SCAN-C 25:76. Laboratory handsheets were prepared asfollowing standard SCAN methods. Filled sheets were produced to achievea grammage of 90 g/m² at a filler level of 25%.

Fine paper machines normally use either alkyl succinic acid (ASA) oralkyl ketone dimer (AKD) to make paper hydrophobic. A level of 0.09%AKD, based oven dry pulp, was added into the ground kraft pulp slurry.In all tests, the cationic starch was added into the pulp slurry firstand mixed well for 30 seconds at 600 rpm with pulp fibers and calciumcarbonate in a Britt Dynamic Drainage Jar (DDJ). At 600 rpm, the effectof shear forces are minimal. The pH was adjusted to 8 with 1N sodiumhydroxide before the filler and cationic retention aids were added intothe pulp. Then, the rotation speed was increased to 2500 rpm for 2minutes in order to simulate shear forces encountered in commercialpapermaking.

In a two component retention system, the cationic substance is normallyadded before the mixing or machine chest in the backwater system of apaper machine. Just before the headbox strong shear forces caused byscreens and pumps break the polymer loops between the anionic substancesof the pulp slurry. The second component is then added before theheadbox in order to increase retention level.

After the DDJ mixing step, pulp slurry was poured into a Noble & Woodssheet former and the pH of the water was adjusted to 8 before slurryaddition. Then, the temperature-activated polysilicic acid agentprepared in Example 1 or Example 2, Compozil, or Hydrocol was added intothe sheet former. One minute mixing with air bubbles in the sheet formerwas applied before the sheet was filtered. The grammage (SCAN-P 6:75)and ash retention (SCAN-P 5:63) levels of the prepared sheets weremeasured. The cationic starch was Hi-Cat 142, sold by Roquette Ltd. Thecationic polyacrylamide was Hydrocol 847, sold by Allied Colloids Ltd.The bentonite was Hydrocol O, and the colloidal silica was BMA 590, soldby EKA Nobel Ltd. See Table 1 for results.

                  TABLE 1                                                         ______________________________________                                        Chemical doses are given as % based on oven dry pulp.                         ______________________________________                                                SiO.sub.2                                                                            Grammage, g/m2                                                                            Ash retention, %                                   ______________________________________                                        Temperature-Activated Polysilicic Acid of Example 2                           1.9% Starch                                                                   1A        0.02     82.5        86.5                                           1B        0.05     82.3        83.5                                           1C        0.13     84.0        85.5                                           1D        0.22     81.3        75.9                                           IE        0.43     85.7        75.8                                           1F        0.54     79.9        73.4                                           1G        0.72     83.6        77.2                                           1H        1.08     83.3        73.9                                           0.6% Starch                                                                   1A        0.02     83.8        84.6                                           1B        0.05     83.8        86.6                                           1C        0.13     84.3        86.5                                           1D        0.22     82.0        75.0                                           1E        0.43     83.4        71.6                                           1F        0.54     83.1        72.9                                           1G        0.72     84.2        71.6                                           1H        1.08     79.8        84.2                                           Temperature-Activated Polysilicic Acid of Example 1                           1.9% Starch                                                                   2A        0.02     83.6        71.6                                           2B        0.05     88.5        90.3                                           2C        0.13     89.6        91.6                                           2D        0.22     90.8        92.2                                           2E        0.43     88.7        91.0                                           2F        0.54     87.8        91.5                                           2G        0.72     89.5        90.7                                           2H        1.08     88.3        91.3                                           0.8% Starch                                                                   2A        0.02     84.6        89.3                                           2B        0.05     87.0        82.5                                           2C        0.13     88.2        93.0                                           2D        0.22     87.5        91.9                                           2E        0.43     87.6        92.0                                           2F        0.54     88.3        92.2                                           2G        0.72     88.2        93.2                                           2H        1.08     88.0        92.9                                           Compozil (colloidal silica sol + cationic starch                              Blank trials                                                                  3A        only pulp,                                                                             61.4         1.30                                                    no filler                                                           3B        pulp and 67.1         9.60                                                    filler                                                              1.9% Starch                                                                   3C        --       75.2        53.1                                           3D        0.36     87.2        92.7                                           3E        0.54     86.5        89.3                                           3F        1.07     85.2        88.7                                           0.8% Starch                                                                   3G        --       78.2        68.4                                           3H        0.07     82.0        84.2                                           3I        0.11     82.3        85.2                                           3J        0.22     81.9        87.1                                           ______________________________________                                        Hydrocol (polyacrylamide and bentonite)                                            Bentonite                                                                              Polyacrylamide                                                                           Grammage, g/m2                                                                          Ash retention, %                           ______________________________________                                        4A   0.27     0.018      75.8      50.9                                       4B   0.27     0.027      76.7      60.2                                       4C   0.27     0.036      77.4      64.7                                       4D   0.27     0.120      86.4      96.0                                       4E   0.54     0.045      79.7      72.9                                       4F   0.54     0.090      83.2      88.8                                       4G   0.54     0.120      84.3      94.0                                       4H   0.72     0.120      84.1      91.4                                       41   --       0.027      72.2      47.8                                       43   --       0.090      77.1      65.4                                       4K   --       0.120      77.7      71.8                                       ______________________________________                                             SiO.sub.2                                                                              Polyacrylamide                                                                           Grammage, g/m2                                                                          Ash retention, %                           ______________________________________                                        Temperature-Activated Polysilicic Acid of Example 2                           5A   0.02     0.120      79.8      77.6                                       5B   0.05     0.120      82.4      80.3                                       5C   0.13     0.120      83.9      82.3                                       5D   0.22     0.120      88.5      83.8                                       5E   0.43     0.120      83.7      77.6                                       5F   0.54     0.120      83.2      76.3                                       5G   0.72     0.120      85.5      76.3                                       5H   1.08     0.120      84.8      77.9                                       Temperature-Activated Polysilicic Acid of Example 1                           6A   0.02     0.120      86.2      88.0                                       6B   0.05     0.120      87.2      91.6                                       6C   0.13     0.120      87.8      92.1                                       6D   0.22     0.120      87.7      93.7                                       6E   0.43     0.120      86.9      88.3                                       6F   0.54     0.120      88.7      92.4                                       6G   0.72     0.120      88.6      92.6                                       6H   1.08     0.120      87.5      91.8                                       ______________________________________                                    

The temperature-activated polysilicic acid of Example 1 showed asignificant improvement over Compozil and Hydrocol. Thetemperature-activated polysilicic acid of Example 2 showed comparableperformance to Compozil and Hydrocol.

EXAMPLE 5 Fine Paper Trial

This example describes the effects of heating on thetemperature-activated polysilicic acid of the present invention andcompares the temperature-activated polysilicic acid with polysilicicacid.

The temperature-activated polysilicic acid was prepared according toExample 1.

The non-temperature-activated polysilicic acid was prepared by diluting60 ml of 3.3 MR sodium silicate solution with 290 ml of deionized water.This solution, which had a silica concentration of about 6% was addedslowly into a solution of 248.5 ml of deionized water and 48.5 ml of 10%sulfuric acid under strong agitation. The final silica concentration was1.5%. The solution was not heated, but was stirred at room temperaturefor 20 hours and then diluted to a 0.15% SiO₂ concentration. The pH ofthe final solution was 1.4.

The polysilicic acid was prepared by adding a thin stream of sodiumsilicate solution with a SiO₂ :Na₂ O ratio of 3.25:1.0 into the vortexof a violently stirred solution of H₂ SO₄ kept at 0°-5° C., stopping theaddition when the pH rose to about 1.7. A 6% solution was made bybringing together solutions of sodium silicate and acid in a zone ofintense turbulence and in such proportions that the mixture had a pHabout 1.5-2.0. This solution was immediately diluted after preparationto obtain a 0.15% SiO₂ concentration. Handsheets were produced using thelaboratory sheet making procedure is as described in Example 3. SeeTable 2 for results.

                  TABLE 2                                                         ______________________________________                                        % Starch  SiO.sub.2                                                                              Grammage, g/m2                                                                            Ash retention, %                               ______________________________________                                        Temperature-Activated Polysilicic Acid of Example 1                           0.8% Starch                                                                   1A        0.09     89.0        91.1                                           1B        0.18     86.6        89.4                                           1C        0.27     85.7        84.6                                           Non-Temperature-Activated Polysilicic Acid                                    0.8% Starch                                                                   2A        0.09     79.9        61.9                                           2B        0.18     86.6        62.6                                           2C        0.27     80.5        58.7                                           Polysilicic Acid                                                              0.8% Starch                                                                   3A        0.09     75.0        56.8                                           3B        0.18     73.1        57.7                                           3C        0.27     75.3        60.1                                           ______________________________________                                    

The temperature-activated polysilicic acid which was prepared by heatingat 60° C. for 20 hours showed a significant improvement in grammage andash retention over both the unheated product and polysilicic acid.

EXAMPLE 6 Fine Paper Trial

This example compares the performance of the temperature-activatedpolysilicic acid prepared in Example 1 (with starch) with that of apolysilicate microgel and Compozil (colloidal silica and cationicstarch). The polysilicate microgel was prepared in accordance with thedescription in Moffett, R.H., On-site Production of a Silica BasedMicroparticulate Retention and Drainage Aid. Papermakers Conference,Book 1, San Francisco, Calif., Marriott, 1994, pp. 243-255. Thepolysilicate microgel was prepared by adding diluted sulfuric acid todiluted 3.3 MR sodium silicate with vigorous mixing to obtain a finalsilica concentration of 2% and a pH of 9. The solution was then mixedfor ten minutes at room temperature. Then, the solution was diluted to a0.15% silica concentration with deionized water. Handsheets wereproduced using the laboratory sheet making procedure was as described inExample 3. See Table 3 for results.

                  TABLE 3                                                         ______________________________________                                        Temperature Activated polysilicic acid of Example 1                           % Starch                                                                              SiO.sub.2    Grammage, g/m 2                                                                           Ash retention, %                             ______________________________________                                        0.8% Starch                                                                   1A      --           80.7        60.6                                         1B      0.036        85.2        79.6                                         1C      0.090        85.5        85.1                                         1D      0.13         89.3        91.5                                         1E      0.18         88.4        90.3                                         1F      0.23         87.2        92.5                                         IG      0.27         86.1        88.1                                         1H      0.36         85.5        84.5                                         1I      0.45         84.0        84.9                                         1J      0.54         82.7        82.8                                         1K      0.63         85.1        81.9                                         Polysilicate Microgel                                                         2A      0.036        82.9        84.8                                         2B      0.090        83.5        87.5                                         2C      0.13         83.7        87.5                                         2D      0.18         84.3        89.5                                         2E      0.23         83.4        90.5                                         2F      0.27         83.3        91.4                                         2G      0.36         86.3        91.6                                         2H      0.45         86.3        91.8                                         2I      0.54         85.7        90.3                                         2J      0.63         84.3        89.3                                         Compozil (colloidal silica and cationic starch)                               0.8% Starch                                                                   3A      only pulp, no filler,                                                                      69.0        1.1                                                  no starch                                                             3B      pulp and filler,                                                                           69.7        10.5                                                 no starch                                                             3C      0.07         86.2        83.7                                         3D      0.14         84.7        80.3                                         3E      0.18         86.4        80.5                                         3F      0.23         85.3        82.6                                         3G      0.27         87.5        83.9                                         3H      0.36         84.9        83.0                                         3I      0.45         86.5        83.6                                         3J      0.54         84.5        84.2                                         3K      0.63         85.4        84.4                                         ______________________________________                                    

The temperature-activated polysilicic acid of Example 1 showedcomparable performance to the polysilicate microgel and Compozil.

EXAMPLE 7 Wood Containing Paper Trial--Comparative Tests

This example compares the performance of four products: thetemperature-activated polysilicic acid of Example 1, thetemperature-activated polysilicic acid of Example 3, Compozil (colloidalsilica and cationic starch) and Hydrocol (bentonite and polyacrylamide).

Unbleached groundwood pulp was used with kaolin. The kaolin was Ckaolin, sold by ECC International. The pulp was received from a Finnishpulp mill as a bale. The groundwood pulp was hot-disintegrated toeliminate the latency of pulp, according to method SCAN-M 10:77.

Laboratory handsheets were prepared as follows, to achieve a 90 g/m²sheet containing 25% kaolin filler.

Wood containing paper machines typically may not use hydrophobic agents.Accordingly, AKD and ASA glues were not used in this example. In alltests, the cationic substance (starch or polyacrylamide) was added intothe pulp slurry first and mixed well for 30 seconds at 600 rpm with pulpfibers and kaolin in a Britt Drainage Jar. At 600 rpm, the effect ofshear forces are minimal. The pH was adjusted to 5 with 1N sulfuric acidbefore the filler and cationic retention aids were added to the pulp.Then, the rotation speed was increased to 2500 rpm for 2 minutes inorder to simulate shear forces encountered in commercial papermaking.

In a two component retention system, the cationic substance is normallyadded before the mixing or machine chest in the backwater system of apaper machine. Just before the headbox, strong shear forces caused byscreens and pumps break the polymer loops between the anionic substancesof the pulp slurry. The second component is then added before theheadbox in order to increase retention level.

After the DDJ mixing step, pulp slurry was poured into a Noble & Woodssheet former and the pH of the water was adjusted to 5 before slurryaddition. Then, the temperature-activated polysilicic acid agentprepared in Example 1 or Example 2, Compozil, or Hydrocol was added intothe sheet former before filtration took place after 1 minute of mixingwith air bubbles in the sheet former was applied before the sheet wasfiltered. The grammage (SCAN-P 6:75) and ash retention (SCAN-P 5:63)levels of the prepared sheets were measured. See Table 4 for results.

                  TABLE 4                                                         ______________________________________                                        Chemical doses are given as % based on oven dry pulp.                         ______________________________________                                        Temperature-Activated Polysilicic Acid of Example 3                           % Starch   SiO.sub.2                                                                             Grammage, g/m 2                                                                           Ash retention, %                               ______________________________________                                        8% Starch                                                                     1A         --      72.1        51.3                                           1B         0.04    74.3        57.4                                           1C         0.09    78.2        68.1                                           1D         0.13    78.7        67.2                                           1E         0.18    76.8        66.3                                           1F         0.23    78.8        70.4                                           1G         0.27    82.1        77.1                                           1H         0.36    76.9        68.3                                           1J         0.45    76.0        63.4                                           1K         0.54    74.7        64.1                                           1J         0.63    72.2        61.9                                           ______________________________________                                        Temperature-Activated Polysilicic Acid of Example 1                               SiO.sub.2                                                                              Polyacrylamide                                                                           Grammage, g/m 2                                                                         Ash retention, %                            ______________________________________                                        2A  --       0.018      72.7      42.1                                        2B  0.04     0.018      70.7      38.3                                        2C  0.09     0.018      71.8      37.7                                        2D  0.13     0.018      72.3      38.8                                        2E  0.18     0.018      71.1      38.8                                        2F  0.23     0.018      72.2      38.9                                        2G  0.27     0.018      70.2      37.5                                        2H  0.36     0.018      69.3      37.7                                        2I  0.45     0.018      67.1      40.8                                        2J  0.54     0.018      67.0      39.0                                        2K  0.63     0.018      66.7      40.9                                        2L  0.09     0.036      72.8      42.5                                        2M  0.18     0.036      71.0      41.0                                        2N  0.36     0.036      71.4      42.9                                        2O  0.54     0.036      70.0      42.1                                        Compozil (Silica sol and polyacrylamide)                                      3A  --       0.036      70.4      42.2                                        3B  0.04     0.018      70.0      40.5                                        3C  0.09     0.018      68.7      38.3                                        3D  0.13     0.018      68.1      38.2                                        3E  0.18     0.018      66.9      36.5                                        3F  0.23     0.018      67.0      37.1                                        3G  0.27     0.018      68.0      37.4                                        3H  0.36     0.018      79.2      36.4                                        3I  0.45     0.018      79.7      41.2                                        3J  0.54     0.018      78.5      38.5                                        3K  0.63     0.018      68.7      39.5                                        3L  0.09     0.036      69.1      40.0                                        3M  0.18     0.036      69.8      41.6                                        3N  0.36     0.036      70.7      45.4                                        30  0.54     0.036      65.1      30.2                                        3P  fibers plus                                                                            --         62.3      8.4                                             kaolin                                                                    3Q  only fibers                                                                            --         53.9      0.3                                         ______________________________________                                        Hydrocol (polyacrylamide and bentonite)                                           Bentonite                                                                              Polyacrylamide                                                                           Grammage, g/m 2                                                                         Ash retention, %                            ______________________________________                                        4A  --       0.018      65.0      37.2                                        4B  0.27     0.009      65.6      39.5                                        4C  0.27     0.018      68.7      41.7                                        4D  0.27     0.036      70.1      46.6                                        4E  --       0.009      66.4      34.8                                        4F  0.54     0.009      69.3      40.9                                        4G  0.54     0.018      70.1      47.2                                        4H  0.54     0.036      70.5      42.9                                        ______________________________________                                    

In this groundwood paper trial the temperature-activated polysilicicacid of Example 3 showed a significant improvement in grammage and ashretention levels over the temperature-activated polysilicic acid agentof Example 1, in which the solution was not mixed at room temperaturebefore dilution, Compozil (colloidal silica and cationic starch) andHydrocol (bentonite and polyacrylamide).

EXAMPLE 8

This example compares the performance of a temperature-activatedpolysilicic acid of the present invention with a colloidal silica.

For the comparative tests, Canadian Standard Freeness measurements weremade according to SCAN-C 21:65. The amount of pulp and filler added intothe DDJ and the handling procedure of the slurry was as describedpreviously in Example 3. However, after mixing at 2500 rpm for 2minutes, the speed was reduced to 600 rpm for 1 minute, when the silicadosage was added into the slurry before the freeness measurements. Thecationic starch was added at a constant dosage of 0.8% based on oven drypulp. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        DRAINAGE MEASUREMENTS                                                         Freeness                                                                                            Temperature-Activated                                   % SiO.sub.2                                                                              Colloidal Silica                                                                         Polysilicic Acid                                        ______________________________________                                        0.05       450        420                                                     0.13       550        570                                                     0.22       530        610                                                     0.43       490        580                                                     ______________________________________                                    

It may be seen from Table 5 that the temperature-activated polysilicicacid agent of the present invention showed activity at least as good asthe colloidal silica and a somewhat enhanced activity at dosages of0.13, 0.22, and 0.43% SiO₂.

It will be understood that various details of the invention can bechanged without departing from the scope of the invention. Furthermore,the foregoing description is for the purpose of illustration only, andnot for the purpose of limitation--the invention is defined by theclaims.

I claim:
 1. A temperature-activated polysilicic acid prepared by heatingan acidified aqueous alkali metal silicate.
 2. The polysilicic acid ofclaim 1, having a pH of about 0 to
 5. 3. The polysilicic acid of claim2, having a pH of about 0 to
 2. 4. The polysilicic acid of claim 1,wherein the aqueous alkali metal silicate is acidified by slow additionof a diluted alkali metal silicate solution into a mineral acid.
 5. Thepolysilicic acid of claim 4, wherein the mineral acid is selected fromthe group consisting of sulfuric acid, hydrochloric acid or phosphoricacid.
 6. The polysilicic acid of claim 4, wherein the aqueous alkalimetal silicate is sodium silicate.
 7. The polysilicic acid of claim 1,wherein the aqueous alkali metal silicate is acidified by slow additionof a diluted alkali metal silicate solution into an ion exchange agentor an organic acid.
 8. The polysilicic acid of claim 1, wherein theacidified aqueous alkali metal silicate is heated at a temperature ofabout 30° to 250° C. for about 1 to 120 hours.
 9. The polysilicic acidof claim 8, wherein the acidified aqueous alkali metal silicate isheated at a temperature of 60° C. for 20 hours.
 10. The polysilicic acidof claim 1, wherein the acidified aqueous alkali metal silicate isheated under pressure to achieve a temperature of about 30° to 250° C.11. The polysilicic acid of claim 1, wherein the heated and acidifiedaqueous alkali metal silicate is mixed at room temperature and dilutedwith water.
 12. The polysilicic acid of claim 11, wherein the heated andacidified aqueous alkali metal silicate is mixed at room temperature for72 hours and diluted to 0.15% SiO₂.
 13. The polysilicic acid of claim 1,having a specific surface area of about 1350 to 1600 m² /g.